1. Field of the Invention
This invention relates to secondary electrochemical cells and more particularly to secondary electrochemical cells utilizing a molten sodium negative reactant, a solid ionically-conductive separator, a molten sulfur and mixture of molten metal or metal-like halides positive reactant, and a molten sodium haloaluminate electrolyte.
2. Description of the Prior Art
It may be explained here that U.S. Pat. No. 3,404,035 discloses a secondary battery that in its fully charged condition has a molten sodium negative reactant, a molten sulfur positive reactant and a solid electrolyte-separator of beta alumina or its derivative disposed between the positive and negative reactants. In its discharged or partially discharged condition, it has an additional electrolyte comprising a sodium polysulfide (Na.sub.2 S.sub.x) on the positive reactant side of the beta alumina separator.
The potential of this battery is typically in the range of from about 2 to 2.2 volts and the operating temperature of this battery is in the range of 265.degree. to 350.degree. C. in order to keep the discharge products, Na.sub.2 S.sub.x, molten at all times. It is recognized by those skilled in the art, however, that operation of this type battery at about or above 265.degree. C. cannot be carried out without attendant problems. For example, Na.sub.2 S.sub.x is a strong oxidizing agent at these elevated temperatures and will tend to effect rapid corrosion and chemical attack of containers, current collectors, seals and separators of the battery, thus seriously reducing the useful life of the battery.
In U.S. Pat. No. 3,877,984, there is described a battery which utilizes a metal chloride positive reactant, e.g. antimony chloride, in contact with a molten alkali metal chloroaluminate electrolyte, e.g. sodium chloroaluminate. The negative reactant of this battery is an alkali metal, e.g. sodium, and a solid ionically-conductive separator, e.g. beta alumina, is positioned between the positive and negative reactants. This battery can operate at a temperature as low as from about 150.degree. to about 225.degree. C. and greatly reduce the corrosion problems since sodium chloroaluminate at temperatures of from about 150.degree. to about 225.degree. C. is not a strong oxidizing agent. Additionally, the voltage potential of this battery approaches or exceeds 3 volts.
In a United States patent application filed on even date herewith entitled "Sodium-Aluminum Halide, Sulfur Battery", which is assigned to the same assignee as the instant application and which is hereby incorporated herein by reference, there is disclosed a novel secondary battery which utilizes a molten sodium negative reactant, a molten sulfur and molten aluminum halide, e.g. aluminum chloride, positive reactant in contact with a molten sodium haloaluminate (molten sodium halide-aluminum halide) e.g. molten sodium chloroaluminate electrolyte. This battery can operate at a temperature as low as from about 150.degree. to about 225.degree. C. and thus also greatly reduces the corrosion and chemical attack problems associated with sodium-sulfur batteries of the type described above with reference to U.S. Pat. No. 3,404,035. The potential of the battery described in this copending application approaches or exceeds 2.66 volts.
One of the differences between the battery described in U.S. Pat. No. 3,877,984 and the copending application just described is that the major discharge products of the battery of U.S. Pat. No. 3,877,984 are antimony metal and sodium chloride, while in the battery of the copending application described above, the major discharge products are, for example, sodium chloride and aluminum sulfide. The antimony metal and sodium chloride discharge products have no deleterious effect on the beta alumina separator or other components of the cell. In early cells constructed in accordance with the teaching of said copending application there appeared to be a continual degradation of the battery with cycling. I speculated that the aluminum sulfide, formed on discharge, attacked the beta alumina separator and/or current collectors of the cell, or at least formed a resistive phase around them.
Working on the assumption tht one of the reaction products of the battery of the copending application described above, namely aluminum sulfide, was attacking the beta alumina, or at least forming a solid resistive layer at its surface, I hypothesized that perhaps by adding a metal, such as for example antimony, to the positive reactant melt of this battery, then the major discharge product would be antimony sulfide or a mixed antimony aluminum salt rather than aluminum sulfide, and thus it would be possible to cycle the cell with no degradation. I subsequently discovered that my assumption was incorrect, the cell of said copending application, in fact, was not degrading in the manner speculated but, I had discovered a novel and, in some respects, a superior cell.
Cells were constructed according to the teaching of the copending application described above, and antimony was added to the positive reactant in chloride form (Sb Cl.sub.3) in the ratio of 5 parts Sb Cl.sub.3 to 2 parts of sulfur. This battery can operate at a temperature as low as from about 150.degree. to about 225.degree. C. The potential of this novel battery ranges between approximately 2.5 and 3.5 volts.